This invention relates to switching and signal generating devices and, more particularly, to magnetically and electromagnetically controlled switching and signal generating devices and circuits therefor.
Conventional reed switches have gained wide use in many industries because of (1) long life, (2) environmental isolation, and (3) repeatable switching performance under variety of ambient conditions. One recent use for reed switches has been as a temperature-sensitive switch, examples of which are disclosed in U.S. Pat. Nos. 3,649,936 and 3,812,441. Such devices are generally comprised of a conventional reed switch with a pair of doughnut-shaped or torroidal magnets mounted over the glass envelope of the reed switch. A ferrite is mounted adjacent to the reed switch and the permanent magnets to form a magnetic circuit with the two permanent magnets.
Ferrites are magnetic materials having a temperature-dependent magnetic permeability and spontaneous magnetization. The properties of ferrites vary abruptly at a temperature denoted as the Curie temperature or point, which is defined as the temperature below which ferrites exhibit spontaneous magnetization and high permeability and the temperature above which these materials lose their spontaneous magnetization and have high reluctance.
Ferrites can be manufactured to have a stable, well-defined, and predetermined Curie temperature, such as between -40.degree. C. and +150.degree. C., by properly setting the mixing ratio, manufacturing temperature, and forming pressure for ferrite materials such as manganese, copper, zinc, iron. Because of the abrupt change in the characteristics of ferrites at the Curie temperature, these materials are useful in constructing temperature-sensitive reed switches of the type discussed hereinabove.
Heretofore in applications requiring a temperature-sensitive reed switch with a different Curie point, it has been necessary to substitute one switch for another which has been manufactured to have the appropriate Curie point. Such switching devices can be used for temperature-sensitive control of such devices as furnaces, water heaters, air conditioners, etc. or as temperature-monitoring devices of various types of electrical equipment, firm alarms, stoves, batteries, nuclear reactor components, machinery, etc.
One example which is illustrative of the problems which have been incurred with prior art temperature-monitoring systems is a warning device for over-temperature (O-T) conditions of aircraft batteries such as might occur during overcharging of the battery. The prior art devices are typically comprised of a temperature-sensitive switch mounted on the aircraft battery and an indicator which is electrically connected to the switch. The indicator is usually positioned at a location remote from the battery, such as in the cockpit of the airplane, to permit the temperature condition of the battery to be monitored in accordance with safety requirements. One problem of this device is that an indicator warning signal can be caused either by an actual O-T condition or a circuit malfunction such as a short circuit, an open-circuit or a mechanical failure of the temperature sensitive switch. Because of this uncertainty, it is desirable to be able to verify that the temperature-monitoring system is operating properly. One solution to this problem has been to merely duplicate the temperature-monitoring system, based on the theory that there is a low probability that both systems will simultaneously fail.
A related problem which occurs with these devices is created by the fact that it is often desirable or necessary to be able to periodically check (e.g. in preflight or bench tests) the accuracy of the temperature-sensing switch in detecting an O-T condition and initiating a warning signal in response thereto. One example of a trial procedure is to supply heat from an external supply to the switch and then measure with a temperature indicator, such as a thermometer, the ambient temperature of the switch contemporaneously with the initiation of a warning signal. If the battery of a heat-conduction path formed therewith is below a predetermined temperature, testing is sometimes difficult because the heat supplied from the external source could be drained off at a rate which would prevent the switch from being actuated. This commonly occurs in in-flight testing due to the fact that the air temperature is normally very low such as, for example, -20.degree. C. To overcome the problem of heat sinking, the most common trial procedure to check the accuracy is a maintenance procedure requiring the periodic removal and replacement of the switch with a substitute so that the original switch can be sent to a lab for testing.
Additional difficulties are also incurred in an in-flight test for either the actuality of an O-T condition and/or the accuracy of detection of an O-T condition. Although not practical, presently known testing procedures would require the reduction of the switch temperature to a temperature less than the switch actuation temperature. This is difficult because the battery, which is attached to the switch, is probably in the indicated O-T condition and thus is a heat source which would tend to maintain the switch in an O-T condition.
An object of the invention is to provide an improved reed switch relay which has an essentially constant power comsumption rate in both the forward and reverse switching modes.
Another object of the invention is to provide a reed switch relay with an essentially hystersis free operation.
These and other objects of this invention will be apparent from a consideration of the detailed description and the accompanying claims.